This invention relates generally to improvements in power distribution systems and, more particularly, to electrical current injection for more effectively reducing non-fundamental currents and voltages in a power distribution system.
The wide use of nonlinear loads, such as those presented by electronic power converters for computers and other electronic equipment, has increased the harmonic content of the voltage and current waveforms in alternating current (AC) power distribution systems. The problem has become especially acute in large office buildings where large numbers of such electronic equipment are operating thereby causing a corresponding increase in power line harmonics. In some of these buildings, the current distortion levels may reach 80%
Such harmonic currents in conjunction with their associated source impedances produce distortion of the line voltages and these distorted line voltages can cause equipment to malfunction. The electromagnetic fields associated with the harmonic currents can interfere with telephone and other communication systems, and the harmonic currents can also result in overheated conductors in conduits and panel boxes and in overheated distribution transformers. Under nonlinear loading, neutral conductors in three-phase power distribution systems, which normally carry insignificant currents for linear loads, are now carrying currents up to approximately 73% greater than the actual line currents.
Prior systems for controlling or limiting the harmonic currents and the distortion voltages they produce have included: (1) placing limits on the amount of harmonic current that loads are permitted to draw by better load design; (2) using passive filters; (3) adding power line conditioners which effectively isolate loads from the power system; (4) running the generating or distribution system below rated capacity to reduce the source impedance and also reduce losses; (5) using zigzag and phase-shifting transformers; and (6) using active filtering techniques.
The technology exists for substantially reducing the harmonic currents drawn by most offending loads through better load design. However, the requisite additional power components and control circuitry add to manufacturing costs, and the costs of retro-fitting existing equipment, such as the large installed base of personal computers, can be prohibitive. Therefore, this approach is not likely to be implemented in the near future.
In-line and shunt passive filters can be designed and installed to remove harmonics that occur at specific frequencies. Passive filters use capacitors and inductors to shunt unwanted harmonic currents. The use of reactive components can provide effective filtering if properly designed and integrated into the power system. However, passive filters generally operate in a narrow frequency band and have some severe disadvantages which often outweigh the advantages. Such disadvantages include catastrophic failure of filter components when unexpected harmonic currents are experienced, degradation of filter performance when load or power source characteristics change, and degradation of the electrical distribution system due to harmonic resonances created by the passive filter itself.
Power line conditioners which utilize AC-to-DC and DC-to-AC tandem power conversion stages isolate load harmonic currents from the input AC line. To be effective in reducing line-side harmonics, however, the input AC-to-DC conversion circuit must use low harmonic conversion technology. Because this type of power conditioner feeds all the load power through two power conversion stages, the power loss is substantial on a relative basis. The power line conditioner is a very expensive solution if the sole function is the reduction of harmonic currents.
Running the system below rated capacity is also an undesirable approach. The system then would not be used to the full extent of its abilities, thus lost capacity results with the associated loss of efficiency and increase in cost.
Zig-zag connections on the secondary windings of three-phase distribution transformers have been used in an attempt to reduce the flow of third harmonic (and other triple) currents through the transformer into the "up-stream" primary circuits. This method of controlling harmonic currents does not remove non-triple harmonics and is less effective when the third harmonic currents are not balanced between the phases. Excessive power loss in the secondary windings due to harmonic currents remains a consideration. Additionally, zig-zag phase-shifting transformers cannot be used for single-phase circuits.
A phase-shifting transformer which outputs two three-phase voltage sets having thirty degrees of phase shift between them can provide cancellation of fifth and seventh harmonic currents in the upstream power circuit. Best results occur when loads having similar fifth and seventh harmonic currents can be equally divided between the two three-phase outputs. The addition of phase-shifting transformers to existing power distribution installations can be an expensive harmonic current solution. In addition to the cost of installing the transformers, load circuits must be divided and connected through separate distribution circuits. Phase-shift transformers also cannot be used for single-phase circuits.
The use of active current injection for the compensation of harmonic currents was proposed in the early 1970's. In the harmonic current injection filter, an injection power source is connected across the AC power line at a point located between the power source and the load and provides a controlled output current. Current to the load is sensed and analyzed, and the harmonic components are input to the injection power source. The injection power source produces the appropriate input-signal-to-output-current ratio to supply the harmonic currents drawn by the load, and therefore the harmonic currents drawn by the load from the AC power source are ideally reduced to zero.
The conventional current injection filter produces an undesirable response, however, in the case where distortion in the AC voltage source is partially responsible for the existence of distorted currents to the load. Due to the inability of prior current injection filters to determine the source of the distorted load currents, such traditional current injection filters will supply the load with the same harmonic currents that the voltage source was supplying thereby "unloading" the distorted AC voltage source for the harmonic voltages. The distorted voltage source may now produce increased harmonic voltages. Similarly, transient disturbances that are normally damped by the load can become undamped by the action of the traditional current injection filter if it is made responsive to transient currents.
Hence, those concerned with reducing the non-fundamental content in power distribution systems have recognized a need for a more effective filter system; one which can reduce current distortion caused by the load without increasing voltage distortion produced by the voltage source. Additionally, there exists a need for providing such a filter system without undue expense and without disturbing the installed base of existing load equipment. The present invention fulfills these needs.